A silica (SiO2) film is an inorganic oxide, and therefore is excellent in heat resistance, electrical insulation, etc., and is also easy to form as a flat film. For this reason, silica film is being used as an electrically insulating film in various fields. Also, silica film is produced by a gas phase method, such as PVD (Physical Vapor Deposition) or CVD (Chemical Vapor Deposition) or a liquid phase method, such as a sol-gel method. A device or electronic component arrangement requiring large electric field strength requires higher insulation, and the silica film needs to be made thicker. However, a thick film is difficult to form by any production method, and cracking usually occurs in a film having a thickness of 1 μm or more. This occurs because the Young's modulus of silica is high (about 105 MPa) and the film cannot follow internal stress produced during film formation due to a difference in the thermal expansion coefficient from the substrate board material or internal stress produced by the shrinkage of the film, and also cannot reduce the stress. Furthermore, in the case of an electronic device having a curved shape or the like, as typified by electronic paper, a silica insulating film cannot follow the deformation because of its low flexibility (high Young's modulus).
In order to solve such a problem, an organic modified silicate film obtained by introducing an organic group into the siloxane skeleton of silica is known, and this is a film of a material also referred to as organic-inorganic hybrids, ormosils, ceramers or the like (see, Sumio Sakka, Sol-Gel Ho no Kagaku (Science of Sol-Gel Method), pp. 115-153, Agne Shofu Sha (1990); and Sumio Sakka, Sol-Gel Ho no Oyo (Application of Sol-Gel Method), pp. 57-115, Agne Shofu Sha (1997)). When an organic group, such as a methyl group is introduced into the siloxane skeleton, rigidity of the siloxane skeleton is reduced and the Young's modulus is decreased, so that even a film having 1 μm or more in thickness can be formed without cracking. This organic modified silicate film is generally produced by a sol-gel method. Using an organoalkoxysilane (RxSi(OR′)4-x, wherein R is an organic group, OR′ is an alkoxy group, and x is 1, 2 or 3) as a starting material, a structure where an organic group (—R) is introduced into the siloxane skeleton can be formed through the hydrolysis of the alkoxy group and the condensation reaction after the hydrolysis. In order to produce an organic modified silicate film having high flexibility, a poly(diorgano)siloxane X-[—Si(R)2—O]m—Si(R)2—X (wherein X is a reactive functional group, and m is the number of organosiloxane units) is used as the starting material and is reacted together with an organoalkoxysilane or metal alkoxide M(OR)N (wherein n is the number of the alkoxy groups and is usually the valence of M). In particular, when the mass average molecular weight Mw is 900 or more, a thick film having 1 μm or more is easily produced and can be an insulating film having flexibility high enough to follow the deformation of the substrate board.
As for a material analogous to the organic modified silicate, in Japanese Unexamined Patent Publication (Kokai) No. 5-239359, a silicone resin composition composed of a chain silicone oligomer such as polydimethylsiloxane, a metal alkoxide and an inorganic filler is disclosed as an insulating coat layer for a heat-resistant insulated wire. Also, Japanese Unexamined Patent Publication (Kokai) No. 2005-79405 discloses, in relation to a thin-film solar cell substrate board, a method for forming an insulating film having formed on the surface thereof an irregular structure so as to enhance the light collection efficiency, where the surface irregular structure is formed utilizing a phase separation into a hydrophobic phase and a hydrophilic phase occurred in the film formed of a polydimethylsiloxane and a metal alkoxide.